This invention relates to an electro-optic scanning device for delivering information-modulated light to a photosensitive surface in an optical information recording apparatus, such as electrophotographic printers and copiers.
It is known to record optical information on a moving photosensitive medium by directing a sheetlike beam of collimated light through electro-optic crystal materials (e.g., lithium niobate or lithium tantalate) while modulating the phase front of the beam to form a scanning line of picture elements or "pixels" across the width of the recording medium. Such known apparatus is disclosed, for example, in U.S. Pat. Nos. 4,367,925 and 4,386,827. As disclosed in these references, the light beam enters a block of electro-optic crystal through a side face thereof, reflects off the base, and exits through the opposite side face. The base of the crystal supports a linear array of closely-spaced and parallel electrodes. Each pair of adjacent electrodes defines one pixel of information. When a voltage is applied between adjacent electrode pairs, fringing electric fields are established within the crystal. Such fields act to selectively alter the refractive index of the crystal, resulting in a phase change in that portion of the phase front of the collimated beam passing through the affected area. Schlieren optics are used to convert the phase-front modulated beam into a corresponding intensity-modulated pixel pattern representing an entire line of image information. By imaging such pattern onto a moving photosensitive recording element while periodically addressing, in parallel, all electrode pairs in the array with line information, a two-dimensional image is produced. Typically, the electrodes and their spacings measure from 1 to 30 microns in width, and it is possible to produce images having over 5000 resolvable pixels per line.
In order for the above-mentioned electro-optic scanning apparatus to produce high quality images, it is necessary that the light-modulating fringing electric fields produced between adjacent electrode pairs deeply penetrate the crystal and be highly uniform across the entire array. Such penetration and uniformity requires a good coupling between each electrode and the underlying portion of the crystal. To achieve such coupling, special care must be taken to polish the crystal surfaces, and special devices, such as elastomeric pads, are commonly used for biasing the electrodes against the crystal. However, even with such polishing and resilient biasing structures, it is difficult to obtain the necessary uniformity in the electric fields to assure that scanline image quality is not degraded. Other disadvantages associated with the above scanning apparatus are: (1) the relatively large electro-optic crystals required by such scanners are costly to produce and, (2) owing to the need to create relatively deeply penetrating fringing electric fields within the crystal, relatively high voltage must be applied between adjacent electrodes.